This invention relates to ceramic materials particularly well suited for use in semiconductor devices having a nonlinear volt-ampere characteristic. The invention also concerns a process for the fabrication of coherent bonded bodies of such materials.
Semiconductor devices with a nonlinear volt-ampere characteristic find extensive use in electric circuits for the absorption of abnormal voltages, suppression of noise, and elimination of sparks. Commonly referred to as varistors, as hereinafter so called in this specification, such semiconductor devices have been fabricated typically from silicon (using the pn-junctions of silicon semicondcutors), silicon carbide (SiC), zinc oxide (ZnO), stannic oxide (SnO.sub.2), and titanium dioxide (TiO.sub.2).
The pn-junction silicon varistors make use of forward voltage rises, so that they offer a varistor voltage of as low as 0.6 volt (V) or so per pn-junction. For higher varistor voltages, therefore, a plurality of pn-junction silicon chips must be interconnected in series. As regards the SiC varistors, the nonlinearity coefficient .alpha. of their volt-ampere characteristic is as low as two or three in a voltage range of 3-20 V. Another disadvantage is that SiC cannot possibly be sintered into annular or other more or less complex shape with sufficient coherency. The ZnO varistors have a high nonlinearity coefficient in a voltage range of over 30 V, but it becomes as low as about two in a low voltage range of 3-20 V. The nonlinearity coefficient of the SnO.sub.2 varistors is higher, being up to about five, in the voltage range of 3-20 V. This advantage is offset, however, by the poor sinterability, expensiveness, and low moisture-withstanding ability of the material. Moreover, since the SnO.sub.2 varistors function as such by making use of the PN-junctions at the interfaces between the electrodes and the semiconductor bodies, their physical properties tend to deteriorate from voltage pulses. The TiO.sub.2 varistors can be molded and sintered into any desired shape, but their nonlinearity coefficient is only two to four in a voltage range of 20-30 V. Additionally, their varistor voltage varies with temperature (temperature characteristic) at a rate ranging from -0.5% to 0.7% per .degree.C., and they are easy to overheat while handling large power.
Another known varistor material being studied is barium titanate (BaTiO.sub.3). The BaTiO.sub.3 varistors utilize the PN-junctions at the interfaces between the electrodes and the semiconductor bodies, just like the SnO.sub.2 varistors, so that they possess drawbacks similar to those pointed out in connection with the SnO.sub.2 varistors.
For the purposes of the absorption of abnormal voltages and the elimination of noise and sparks, the parallel connections of varistors and capacitors have hitherto served better than varistors only, as is well known to the specialists. No ceramic composition having both functions has so far been discovered, as far as the applicant is aware. Such a material will significantly contribute to the miniaturization and cost reduction of electrical equipment.
U.S. Pat. No. 3,933,668 issued to Takahashi et al. on Jan. 20, 1976, describes and claims a ferroelectric ceramic compound in a perovskite structure, composed principally of strontium titanate (SrTiO.sub.3). This known ceramic material is well suited for use in capacitors. The high resistance offered by the particles constituting the ceramic, however, practically inhibits its use in varistors. Another disadvantage of the known SrTiO.sub.3 ceramic is easy deterioration of electrical properties due to voltage pulses